Francisella tularensis subspecies tularensis, also known as type A Francisella, is a category A priority pathogen for Biodefense Research. It causes a potentially life-threatening disease called tularemia. F. tularensis is a low dose pathogen;infection with less than 10 organisms can cause a lethal infection. A novel F. tularensis lipoprotein, designated Francisella infectivity potentiator B (FipB) in the type A Schu S4 strain has been shown to be essential for virulence. The goal of this proposal is to characterize the role of FipB in virulence and identify critical aspects of F. tularensis pathogenicity. Because FipB is essential for virulence in a type A strain, the experiments described in this proposal will have a significant impact on our understanding of the processes and pathways of F. tularensis that are essential for virulence. FipB has several features that can be used to investigate its function in virulence. As a lipoprotein, it has been shown to stimulate the innate receptor TLR2. Bioinformatic analyses indicate that FipB two conserved domains. One domain shares sequence and structural similarity with a class of virulence proteins called Mips (macrophage infectivity potentiator). The other domain is the conserved enzymatic active site of the DsbA class of oxidoreductases, that have been shown to be required for the formation of virulence factors in other Gram-negative bacteria. Specific deletions and site-specific mutations of fipB will be created in order to define the regions and features of FipB that are required for internalization and intracellular survival. Identifying the critical functional domains of FipB will aid in development drugs that target the critical regions of this novel protein;characterizing the virulence pathway may also identify additional targets for new therapies.